Inflammatory joint diseases, such as psoriatic and rheumatoid arthritis, involve a number of debilitating symptoms for those who suffer from them. The underlying reasons for these symptoms include bone and cartilage degradation. This degradation occurs through bone resorption, which is a process wherein osteoclast cells degrade existing bone. The present application discloses that the increase in osteoclast cells is preceded by an increase of osteoclast precursor cells (OPCs) in the peripheral blood of a subject with an inflammatory joint disease. Disclosed are methods of diagnosing inflammatory joint disease, as well as treatments for inflammatory disease, among other methods and compositions related to osteoclasts and osteoclast precursor cells and inflammatory joint diseases.